The invention relates to a level control system for a motor vehicle, by means of which a vehicle body is spring-suspended relative to at least one vehicle axle and which comprises the following components:                a compressor with an inlet and an outlet,        air springs, each of which is connected to the outlet of the compressor via a compressed air line, which may be shut off by a switchable directional control valve,        at least one pressure sensor, which is connected at least from time to time to the outlet of the compressor via a sensor compressed air line.        
Such level control systems are known from the prior art, e.g. from EP 0 978 397 B1. In the level control system described therein, the air springs are filled solely from the atmosphere or emptied solely thereinto (open level control system). A level control system of the abovementioned type is likewise known from EP 1 243 447 A2, which, in addition to the abovementioned components, has a compressed air accumulator from which the air springs are filled or into which they are emptied (closed level control system). After initial fitting of a level control system in the motor vehicle, the air springs thereof are filled with the assistance of the compressor. In the case of a closed level control system, this takes place for instance in that first of all the compressed air accumulator is filled via an external connection of the level control system. Then, compressed air is transferred from the compressed air accumulator into the individual air springs by means of the compressor. It is assumed during this operation that the compressor is correctly connected and operating faultlessly. Faulty connection of the compressor is therefore only recognized after a time delay, specifically from the fact that the air springs cannot be filled as desired. The time delay may amount to up to a minute or more, which is a long period of time when it comes to identifying a fault on a modern production line. It should additionally be noted that faulty installation of the compressor is identified only at an advanced point on the production line. This may be disadvantageous with regard to further assembly on the production line.
Later on, damage to the compressor may arise during servicing or repair of a level control system. In this instance too, there is no quickly performable test for checking that the functional soundness of the compressor is not reduced.
The object of the invention is to provide a level control system in which the compressor may be checked simply in a short time for problem-free readiness for service.